Both 'our' Kirk and Mirror Spock knew the Mirror Halkans were toast. Once Mirror Kirk was back in charge, that was inevitable. But Mirror Spock also knew he had Mirror Kirk's ear, and that was all he needed to begin to make changes. Even something as pragmatic as getting Mirror Kirk to spare some Halkans, maybe a city or two worth, to work the mines was doable. Using local labor is always cheaper.

Well, considering that "our" Spock was sending the Mirror Kirk/Scott/McCoy/Uhura through at the same time, if "our" group had stayed in the MU I think their lifespans could have been measured in milliseconds after that.

We now await someone to come in here and say our Kirk violated the Prime Directive in sparing the Halkans and trying to affect change in the Mirror Universe.

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On the contrary, I'd like to present this dialogue from "Ensign Ro" (TNG):

PICARD: Then I don't understand why you are unwilling?
KEEVE: Because you are innocent bystanders. You were innocent bystanders for decades as the Cardassians took our homes, as they violated and tortured our people in the most hideous ways imaginable, as we were forced to flee.
PICARD: We were saddened by those events but they occurred within the designated borders of the Cardassian Empire.
KEEVE: And the Federation is pledged not to interfere in the internal affairs of others. How convenient that must be for you, to turn a deaf ear to those who suffer behind a line on a map.
PICARD: Well, I'm not here to debate Federation policy with you, but I can offer you assistance.

Assuming this aready was Federation policy in the 23rd Century, I'd say that everything that happened in the alternate universe of "Mirror, Mirror" was "the internal affair of others".

Both 'our' Kirk and Mirror Spock knew the Mirror Halkans were toast. Once Mirror Kirk was back in charge, that was inevitable. But Mirror Spock also knew he had Mirror Kirk's ear, and that was all he needed to begin to make changes. Even something as pragmatic as getting Mirror Kirk to spare some Halkans, maybe a city or two worth, to work the mines was doable. Using local labor is always cheaper.

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I though Mirror Kirk was pretty much sentenced to death already due to apparent waffling over his Halkan orders when Kirk was acting in his place.

I mean, he might very well have revolted, escaped, etc., but Mirror Spock couldn't count on achieving much with Mirror Kirk even if he did have his ear.

Or was the Empire flexible enough to say "All right, we're rescinding your execution this time! It's not worth the trouble!" as long as Mirror Kirk finally followed orders and killed the Halkans?

The Empire hadn't sentenced MUKirk to death. It was MUSulu that did that, and look how that turned out. MUSulu may have sent word to the Empire as to why he was acting, but once MUKirk was back, and with MUSulu having spent some time in the booth for insubordination, the Empire would have been dealing with MUKirk, with all that entails.

So MUSpock now has a choice. He can do nothing, and nothing changes, or he can come up with a plan for changing the future. What we saw in DS9 was one version where he came up with the wrong solution, one that saw a violent end to the Empire, and the subjugation of its member races.

From what little we actually see of mirror Kirk, it seems clear that he is basically an inhuman monster who would never follow Spock's plans to reform the Empire.

As for the fall of the Empire...

Spock planned that all along. He wanted the Empire to fall, so humans would see what it's like to be oppressed - and thus make sure it never happens again. Spock had the whole thing planned out from the get-go. He would, wouldn't he?

Why would Kirk and co ever want to live in the Mirror Universe?
Just for the slim chance of saving the Mirror Halkans.

The rest of their lives would be hell watching their backs all the time.
And then in 'their universe' their Mirror counterparts would probably cause real mayhem pillaging and destroying civilisations and gaining power using their identities.

Why would Kirk and co ever want to live in the Mirror Universe?
Just for the slim chance of saving the Mirror Halkans.

The rest of their lives would be hell watching their backs all the time.
And then in 'their universe' their Mirror counterparts would probably cause real mayhem pillaging and destroying civilisations and gaining power using their identities.

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Actually that's highly unlikely. Our Spock already figured out where they came from. If there were no way to return them he certainly would take steps to minimize or eliminate any damage they could cause. And Spock stated that the Mirror counterparts were lousy at hiding what they were because they had no examples of civilized behaviour to draw from.